Managing Upstream Transmission in a Network

ABSTRACT

A bandwidth allocation and monitoring method may divide available bandwidth on a shared communication medium into a plurality of discrete tones that can be individually allocated to modems on an as-needed basis. The effective modulation rate that a particular modem can use for each discrete tone can be monitored over time using a schedule of pilot tones transmitted from the modems on different tones at different times. The schedule may define representative pilot tones, in which case effective modulation rates for neighboring tones may be inferred from a determined effective modulation rate of a pilot tone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to co-pendingU.S. application Ser. No. 14/482,765, filed Sep. 10, 2014 and is acontinuation of and claims priority to U.S. application Ser. No.12/776,584, filed May 10, 2010, and entitled “MANAGING UPSTREAMTRANSMISSION IN A NETWORK.” The above-mentioned application isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The features described herein generally relate to managing access to ashared transmission medium, such as upstream transmissions in a contentdistribution or data network.

BACKGROUND

Conventional data networks, such as hybrid fiber coaxial cable networks,optical fiber or coaxial cable networks, wireless networks, satellitenetworks, and the like, allow one or more devices to communicate withone another, often times sharing access to a common transmission medium.In the example network depicted in FIG. 1, a central office 100 (e.g., acontent provider, headend, etc.) may be connected to a distributionnetwork over nodes 101 a-b. Various homes 102 a-f may be connected tothe network, and may receive signals carrying content from the centraloffice 100. The content may be, for example, television programming andmovies received from content providers, stored at the central office 100or received from another source using an external network (e.g., anInternet Protocol backbone network, a satellite network, etc.). Thesignal loss over fiber optic cable is much less than the loss overcoaxial cable, thus the fiber optic connection between the centraloffice and the fiber node 101A is often very long (e.g., 10 to 40 km).The signals over coaxial cable are much easier to split and amplify andterminate than fiber optic cable, thus the final connection from thefiber node 101A to the customers home devices is typically done withcoaxial cable.

The central office 100 may offer downstream signals carrying thisinformation (as illustrated by the arrows in FIG. 1). The central office100 may also receive upstream communications from the various homes 102a-f. For example, in one type of system under the Data Over Cable SystemInterface Specification standard (DOCSIS), the available bandwidth onthe distribution network is divided by frequency range into a downstreamportion and an upstream portion. Most of the frequency allocation is inthe downstream direction (e.g., 54-860 MHz), while a smaller portion(e.g., 5-42 MHz) is allocated for upstream transmissions. Since thevarious homes 102 a-f need to share the transmission medium for theirupstream transmissions, the DOCSIS standard calls for that sharing to becontrolled by a termination system, such as a Cable Modem TerminationSystem (CMTS) at the central office 100. Under DOCSIS, the CMTSinstructs the various homes 102 a-f (or, the cable modems in thosehomes) on when they can use the upstream bandwidth.

The centralized approach in DOCSIS allows for the centralized managementof the upstream bandwidth, but it has a disadvantage. Upstreamtransmissions to a given CMTS are carried on a selected frequencychannel within the 5-42 MHz upstream allocation, but the entire 5-42 MHzrange is not used by all homes, because some homes suffer interferencewithin that range. For example, home 102 a may be located near a sourceof electromagnetic interference in that range, while home 102 f may needto traverse a longer geographic distance, and pass through moresplitters 101 a-c, to reach the CMTS (resulting in greater loss ofsignal in different ranges), suffering interference along the way(particularly at the lower ends of the upstream range). Other homes,however, might not suffer these drawbacks. Some homes 102 d might begeographically closer to the central office 100 and its CMTS, some 102b-c may have fewer intervening splitter nodes, and others 102 e may havean intervening fiber optic portion 104 that allows signals to avoid muchof the interference that would otherwise be suffered along the path ofthe fiber optic cable. Some of these homes may be able to use otherfrequency portions that are not within a chosen DOCSIS upstream channel.

There is an ever-present need to offer greater data transmissioncapability to end users.

Furthermore, the upstream channel may be nonlinear, and that may presentissues for transmission. Most other media for communication are linear,such as twisted pair, satellite, and wireless. In hybrid fiber coaxial(HFC) networks, for example, the conversion from coaxial cable to fiberoptic cable in the upstream requires an optical laser transmitter to beplaced in the transmission path between modem transmitter and modemtermination system receiver. This leads to nonlinear distortion in theHFC upstream channel that is not present in most communications systems.Also, since the upstream spectrum allocation is so low in frequency,5-42 MHz, there is very little signal attenuation in the upstream. Mostcommunication systems have a flat noise response over the communicationschannel such as wireless systems or a noise response that gets worse athigher frequencies such as DSL. In these systems the primary noisedeterminant is the signal attenuation and the receiver noise floor. Inthe HFC upstream, the noise is worse at lower portions of the spectrumsince the noise level tends to decrease with higher frequency but thesignal level stays relatively flat. Since the upstream is amplifiedoften in the HFC upstream path and the fiber optic link has very littleattenuation, the noise floor is not determined by signal attenuation andreceiver noise floor. In the HFC return path channel, thesignal-to-noise ratio tends to improve as the frequency increases and isfundamentally determined by external noise sources, upstream modemtransmit power limitations, return path amplifiers and filters, and thenoise and distortion characteristics of the optical return path lasertransmitter in the fiber node.

SUMMARY OF THE DISCLOSURE

Features described herein allow for an alternative approach to managingupstream bandwidth, and one that may offer greater bandwidth capacitiesand at higher reliability than is currently available. In someembodiments, the modems may transmit over the entire available upstreamspectrum, thus fully utilizing the spectrum dedicated to upstreamtransmission. Many modems can share the upstream spectrum using acombination of time division, frequency division, and code division. Thespectrum may first be divided into sub-channels, with each sub-channelincluding three types of tones: data tones, ranging tones, and pilottones.

Use of the tones may be managed by a centralized server or modemtermination server, such as the CMTS in DOCSIS. The central server mayprovide the various modems with a map identifying the various tones.Along with the map, the central server may provide the modems with aschedule, according to which the modems may transmit a pilot signal onone or more of the tones. The schedule may require each modem toprogressively step through the various tones, transmitting pilot signalson each tone for a predetermined amount of time, so that over a longerperiod of time, the modem will have eventually transmitted pilot signalson all (or a defined subset) of the possible tones. Viewed over time,the subset of tones carrying pilot signals may be a moving subset,stepping through the various tones.

The centralized server may record the various results of the pilotsignals from each modem, and measure distortions, and it may transmitacknowledgement and predistortion signals back down to each modem,informing the modem that the last pilot signal was received and howfuture transmissions on the tone should be predistorted. With thisfeedback, each modem may adjust the symbol/modulation rate it would useon that particular tone, and the centralized server may record themaximum data rate that a particular modem can achieve when transmittingon a particular tone. The server may then use this information todetermine how best to allocate tones when modems need to transmitupstream data, and the adaptive modulation may allow for the mostefficient use of the various tones. The adaptive modulation and pilotsignal information may also allow the modem to pre-distort upstreamtransmissions to account for linear and nonlinear distortion occurringon the tone.

In some embodiments, the pilot signal schedule for a given modem mayresult in the modem continuously transmitting pilot data, hopping thetransmission from tone to tone according to the schedule. Requests forupstream tone allocation may be added to the pilot signal, allowing thepilot signals to be multi-purpose. Additionally, this hopping mayintentionally skip some tones, allowing for a given pilot signal on atone to also define characteristics of neighboring tones.

These features, and others, may be offered using one or more of theembodiments described in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a data distribution network on which the variousfeatures described herein may be implemented.

FIG. 2 illustrates a computing device that may be used to implement thevarious components described herein.

FIG. 3 illustrates an example frequency distribution map showing thevarious tones that may be defined using features described herein.

FIG. 4 illustrates example pilot and ranging signals.

FIG. 5 illustrates a frequency map over successive symbol periods.

FIG. 6 illustrates contents of an example adaptive modulation tone map.

FIG. 7 illustrates an example process of managing discrete tones.

FIG. 8 illustrates an example process of a modem using tones describedherein.

FIG. 9 illustrates an example frequency map that includes accommodationsfor backwards compatibility.

FIG. 10 illustrates an example process of handling distortions in thesignal path between devices.

FIG. 11 illustrates an example nonlinear ranging process to measuredistortion.

DETAILED DESCRIPTION

FIG. 2 illustrates an example computing device 200 that can be used toimplement the various structures and components described herein, suchas a gateway or interface device at a user's home, a modem, and a modemtermination server. The device 200 may include one or more processors201 that may execute software programs or instructions to performfunctions described herein. The programs and instructions may be storedin a storage 202, which may be any desired type of computer-readablemedium. For example, storage 202 may be a hard disk, floppy disk,compact optical disk, FLASH memory (internal and/or removable), or anyother desired type of storage. The device 200 may include an audio/videooutput circuit 203, which may be any desired type of audio and/or videocircuitry that can send output to a device 204, such as a monitor. Forexample, the audio/video circuit 203 may include an HDMI (HighDefinition Multimedia Interface) interface circuit, component videooutput, VGA (Video Graphics Array) video output, stereo audio output,textual information displays, etc.

The device 200 may also include user interface circuitry 205 that canprovide any desired interface to a user. The interface may includedisplays (such as the monitor 204), LED (light-emitting diode) displays,pushbuttons, microphones, infrared sensors, and/or any desired type ofuser interface device that will allow a user to interact with the device200.

The device 200 may also include connectivity for networks. One or moreexternal network interfaces 206 may be included to permit the device tocommunicate with external networks that are off-site (e.g., outside ofthe premises on which the device 200 resides). These interfaces mayinclude, for example, hardware and/or software for cable modems, fiberoptic interfaces, twisted-pair telephone wire interfaces, wirelesscellular interfaces, wireless mesh network interfaces, WiMAX (WorldwideInteroperability for Microwave Access) interfaces, satellite interfaces,and any other desired type of external network interface.

The networks need not be external to a premises of the device 200. Thedevice 200 may include one or more local network interfaces 207 to allowcommunications with on-premises networks as well. Such local networkinterfaces can include local wireless (e.g., IEEE 802.11) interfaces,local wired (e.g., Ethernet) interfaces, and any other desired interfacefor an on-premises network.

FIG. 3 illustrates an example frequency map for the upstream portion ofa network, illustrating the upstream frequency range (e.g., 5-42 MHz)for a time period of 100 microseconds. This frequency range may bedivided into a number of discrete ranges, or tones. For example,individual tones may be defined at a 10.51136 kHz spacing, resulting intones that occupy approximately 10.5 kHz bandwidth. Using this tonesize, the 37 MHz overall upstream range depicted (5-42 MHz) can bedivided into approximately 3,520 tones (37 MHz/10.51135 kHz). Theright-hand side of FIG. 3 shows an expanded portion of the overallrange, depicting 18 of these tones occupying a 189.2 kHz portion of thespectrum. As will be described below, each of these tones may beindividually allocated to different modems, depending on how well theparticular modem is transmitting on that tone, and on the modem'stransmission needs.

In general, a modem may transmit data on a particular tone to a centralcontroller (which may, in one type of network, be a CMTS implementedusing a computing device as in FIG. 2). The modulation used by the modemon that tone governs how much data can be sent. For example, ifQuadrature Amplitude Modulation (QAM) is used to send symbolsrepresenting data on the tone, then 256 QAM can convey an 8-bit quantityfor each symbol. If there is too much noise or interference on that tonefor that modem to adequately transmit 256 QAM, then it may use alower-level modulation, such as 64 QAM, 16 QAM, or simple QuadraturePhase Shift Keying. The lower-level forms of modulation convey lessinformation, but are more tolerant to noise and interference.

To determine which modulation scheme a modem can use on a particulartone, the modem can be instructed to transmit a test signal, or pilotsignal, on that tone using a predetermined modulation. FIG. 4illustrates an example of a pilot tone 401 carrying a pilot signalpacket 402. The pilot signal packet may contain data 402 a identifyingthe transmitting modem and a test data pattern 402 b that the receiveris expecting to receive. In some embodiments, the pilot signal packetmay also include an allocation request 402 c. The allocation request 402c will be discussed in greater detail further below. FIG. 4 alsoillustrates some additional guard time 402 d that may be placed in thepacket to allow microreflections to settle.

The transmission of the pilot signal packet may be made according to apredetermined modulation scheme. The pilot signal may measure thechannel conditions, such as signal to noise ratio and symbol error rateby demodulating the pilot with a known symbol pattern and modulationrate. For example, the modem might first use the highest supportedscheme (e.g., 256 QAM) for the transmission for a data tone if the pilottone measurement has a high signal to noise ratio and a low symbol errorrate. If the pilot signal is successfully received at the centralizedserver, then the modem is confirmed as being able to use 256 QAM on thattone. If the pilot tone signal measures a low signal to noise ratio anda high symbol error rate, then the modem may need to step down themodulation, choosing a lower-bandwidth, more noise tolerant, scheme andtry again. This process is described in greater detail further below.

Over time, and according to a predetermined schedule, a given modem mayeventually transmit pilot signals across all available tones, with eachpilot signal being used to test whether a given modulation is effectiveat that tone for that modem. If a modem's pilot signal is notsuccessfully received at the central controller, then the modem mayretry the pilot signal, but at successively lower levels of modulation,until the pilot signal is successfully received at the centralcontroller.

Some tones may also be used as a ranging tone to carry a ranging signal403. Ranging signals may permit a central controller to detect andquantify distortions on signals transmitted from a modem caused by theintervening cable network between the central controller and modem.Example distortions include time delay (time from transmission toreceipt), amplitude distortions (shifting in amplitude) and phasedistortions (shifting in phase). The central controller may respond tothe modem with the characteristics of the ranging signal that wasreceived, and the modem may determine the timing delay andamplitude/phase distortions that occurred (e.g., by comparing thereceived information with time/amplitude/phase information identifyingthose characteristics of the ranging signal that the modem sent).Alternatively, the central controller may perform these calculations, inwhich case it would receive information from the modem identifying thecharacteristics of the original signal that was sent. The distortionsmay be determined for the base tone, and also on harmonics of the basetone, to allow the modem to compensate.

This compensation may be done, for example, by having the modem addpre-distortion to the transmission signal to cancel out the distortion.In this manner, pre-distortion increases the signal to noise ratio andthe signal to interference ratio in the upstream channel, therebyincreasing the channel capacity and allowing the channel to supporthigher orders of modulation. The ranging signal may simply be atransmission of the base carrier frequency for the tone (e.g., with nodigital data modulated onto the tone). Alternatively, data may bemodulated onto the ranging signal (e.g., identifying the transmittingmodem, identifying characteristics of the ranging signal being sent,such as original time of transmission, amplitude and/or phase), and thereceiving central controller would need to account for this data whendetermining the distortion/predistortion. In some embodiments, the pilotsignals may be used as ranging signals as well.

The process of sequentially testing the various tones may help thecentral controller build a comprehensive table identifying themodulations level(s) that each modem is able to use on each tone in thesystem. Having each modem test each tone individually may be the mostaccurate approach, but it may also be processing intensive for thecentral controller to be monitoring so many pilot/ranging signals (e.g.,the example above lists 3520 tones in the 37 MHz upstream range). Tohelp streamline the testing and monitoring at the central controller,some embodiments may group the tones for testing purposes, and use pilotand ranging tones to represent characteristics of their neighboringtones. For example, a single pilot tone might be used to represent 108other neighboring tones, on the assumption that the neighboring toneswill share similar interference and signal characteristics. Similarly, asingle ranging tone might also be used to represent 108 otherneighboring tones, on the assumption that ranging and distortion for theneighboring tones will be similar to that of the ranging tone. With suchgroupings of 110 tones (108 data tones, one pilot tone, one rangingtone), the 3,520 available tones in the example above may be checkedusing 32 pilot tones and 32 ranging tones (3520/110=32).

As discussed above, the modems may transmit pilot and ranging signals ontones to allow the central controller to assess the modem's ability touse that tone. Each modem may transmit these signals on different tonesat different times. FIG. 5 illustrates example frequency maps for fourtime periods, illustrating how the pilot and ranging tone transmissionscan hop through tones over time. As illustrated, the modem may transmitpilot tone 501 a and ranging tone 502 a on the two tones illustratedduring the first time period. In successive time periods, the modem maytransmit its pilot tone 501 b,c,d and ranging tone 502 b,c,e ondifferent tones in the frequency map. Within each time period, thecentral controller may receive these pilot and ranging tones, and maydetermine the modem's ability to use those tones (as will be describedin greater detail below). The transmission schedule may result in themodem transmitting a pilot and/or ranging tone every symbol period, sothat each modem is always transmitting one of these test signals.

The duration of the illustrated time periods in FIG. 5 may be a singlesymbol period (e.g., long enough to transmit a single symbol using adesired modulation, such as 256 QAM). For example, a 100-microsecondsymbol period may be chosen. That rate allows for 10,000 symbols persecond, and if 256 QAM is used as the modulation, then a given tone cansupport a data rate of 80,000 bps. Following the example having 3520tones grouped into 110-tone groups, and assuming that the 32 pilot and32 ranging tones in each group are not used to carry data, then the 256QAM supports a total upstream bandwidth of 276.48 Mbps ((3520 totaltones−64 pilot/ranging tones)*80,000 bps/tone). The 100-microsecondsymbol period is also convenient for alignment with symbol periods oflegacy modems, such as DOCSIS 1.0 and 2.0 modems, as will be discussedfurther below.

The FIG. 5 illustration shows only four time periods, but many timeperiods may be used to allow the modems to eventually transmit the pilotand/or ranging signals on all of the tones in the system. Additionally,a modem may transmit a pilot or ranging signal on a first tone at onetime period, and then in a later time period, it may transmit on thesame tone again. Such retransmission may be useful to confirm that themodem is still able to use that tone, or to try a different modulationlevel to determine whether a lower/higher data modulation rate issupported by the modem on that tone.

The actual manner in which a modem steps through the various tones mayvary. In some embodiments, the central controller may transmitinstructions to the various modems, informing them of which tones to usefor pilot and/or ranging signals, when to use those tones, and whatmodulation to use on those tones. These instructions may result in themodem transmitting a pilot and/or ranging signal during every timeperiod. Such continuous transmission (although across different tones)of the pilot signal may, for example, allow the modem to have animmediate mechanism for transmitting an upstream signal (such as arequest for tone allocation) to the central controller.

In other embodiments, the central controller may transmit a schedule oralgorithm that may be used to dynamically determine when the next pilotor ranging signal should be sent, and on what tone. The schedule mayindicate times and tones, while the algorithm may include dynamicinstructions. For example, the instructions may instruct the modem torepeat a given tone for a number of times (e.g., ten times), and if thesignal is successfully received at a given modulation 9 out of 10 times,then try the next higher level of modulation. The conditions forstepping down may be based on the desired effective rate. For example,the modem might try a given modulation ten times, and lower themodulation if more than 1 fails. The adaptation map can be updated toindicate the next modulation that should be tried on the next pilotsignal sent by the modem.

The ranging tones may measure the time delay, frequency offset,attenuation, AM to AM and AM to PM non-linear distortion for eachfrequency slot in the 5-42 MHz upstream band (or whatever the upstreamspectrum allocation is, not necessarily always 5-42 MHz). The rangingtone measurements are not expected to vary significantly over time orfrequency from observations of upstream channel characteristics. Thusthe sequence of ranging tones is not critical to operation and so thesub-channel of a given modem may be assigned based upon other criteriadescribed below and the tone assigned for ranging within the sub-channelcan linearly progress from lowest frequency to highest frequency andthen repeat.

In some embodiments, there may be just one ranging tone per sub-channeland only one modem per ranging tone. Within an example sub-channel thereare 110 possible center frequencies available for the tones. When amodem is assigned a ranging tone within a sub-channel for the firsttime, the lowest frequency may be assigned, f_subchannelx_0. The nexttime this modem is assigned a ranging opportunity on that sub-channelthen the ranging tone center frequency may be the next highest frequencytone, f_subchannelx_1. This process may continue until the highestpossible center frequency within the sub-channel is reached,f_subchannelx_109.

The next ranging tone within the sub-channel in the sequence can bef_subchannelx_0. The ranging process can use many symbols, and a fullranging process may vary in terms of the number of symbols required tocomplete ranging. For non-linear ranging, multiple harmonic rangingtones can be used.

In general as many modems will range as possible and over a long periodof time. Each modem can have an equal number of ranging opportunities.For example, with 128 modems served and 32 ranging tones, the modems canbe grouped into four sets of 32 modems, linear_ranging_group_1,linear_ranging_group_2, linear_ranging_group_3, linear_ranging_group_4.First, members of linear_ranging_group_1 can each be assigned adifferent ranging tone and sub-channel, and they can complete the fulllinear ranging process. Then members of linear_ranging_group_2 cancomplete the linear ranging process, and so on until the first groupwill complete another ranging process. Non-linear ranging can beperformed one modem at a time, and thus with 128 modems each modem cantake a turn at nonlinear ranging every 128^(th) non-linear rangingopportunity.

Pilot tones can measure the noise and interference levels within eachsub-channel and tone bandwidth. The noise and interference levels mayvary significantly over time and frequency. For this reason there aretimes when pilots should be concentrated in frequency bands that havedemonstrated a history of varying interference. These tend to beconcentrated in the lower portion of the spectrum and at frequency bandsused in radio communications. In one example configuration, there are 32sub-channels and 1 pilot per sub-channel and 4 modems per pilot tone.Thus 128 modems can send a pilot tone at the same time. If only 64modems are served, then each can send multiple (e.g., 2) continuouspilot tones, since it is desirable to send as many pilot tones aspossible to get a better measurement of channel conditions overfrequency and time.

The modems served can be randomly and evenly distributed over 32sub-channel assignments groups, such as groups going from zero to 31.For example, modem_0, modem_1, modem_2, modem_3 can be assigned tosub_channel_assignment_group_0 while modem_124, modem_125, modem_126,modem_127 can be assigned to sub_channel_assignment_group_31.

The lowest frequency sub-channel can be designated as sub_channel_0, andthe highest frequency sub-channel can be designated as sub_channel_31.The four modems in sub_channel_assignment_group_0 can be first assignedto sub_channel_0 for enough symbol periods to transmit a pilottransmission (e.g., eight symbol periods may be needed in someembodiments). Then, the four modems in sub_channel_assignment_group_0can be assigned to sub_channel_1 for the next eight symbols. This cancontinue until sub_channel_assignment_group_0 is assigned tosub_channel_31, and after that sub_channel_assignment_group_0 can beassigned to sub_channel_0 again and the process can be repeated.

Likewise, sub_channel_assignment_group_1 tosub_channel_assignment_group_31 can step through the sub-channels.Within the sub-channel, the pilot tone center frequency can linearlyprogress from lowest frequency to highest frequency. However, over timepatterns of interference can be recorded that will alter the assignmentof pilot tone center frequencies within a sub-channel. Once astatistical measurement of the interference level within a sub-channelhas been measured, then the assignment of pilot tone center frequencywithin a sub-channel can be based upon the level and probability ofinterference. For example, if during the hours of 6 pm to 8 pm a groupsof tones within a sub-channel has a 90% probability of interferenceabove a threshold level while the other tones have a 10% probability ofinterference above a threshold level, then the pilot tones can beassigned 9 times to the groups of tones suffering high interference forevery 1 time that the pilot is assigned to a group of tones with ahistory of less interference.

Data tones can be assigned to modems based upon upstream bandwidthrequests. If only one modem has requested upstream transmission, thenall data tones can be assigned to that modem. In the illustrativeexample, if all 128 modems request upstream transmission, then a subset,such as 27 data tones, can be assigned to each modem. Higher levelquality of service metrics can be applied to determine how quickly abandwidth request from upstream transmission is assigned data tones.

As the various modems in the system transmit their pilot and/or rangingsignals, the central controller may store information identifying signalquality characteristics on a per-modem and per-tone basis. FIG. 6illustrates an example adaptive modulation map that the centralcontroller may have access to, or record in a memory. For eachdevice/tone combination, the adaptive modulation map may store toneusage information for that device (e.g., modem) and tone. For example,the tone usage information may include information identifying a statusof the particular tone and device (e.g., whether the device is active),the particular modulation that the device should use on that tone (e.g.,256-QAM), the amount/type of forward error correction (e.g., errorcorrection codes, predistortion, etc.), the overall data rate on thetone (which may simply be the modulation rate, or may take into accountother factors).

FIG. 7 illustrates an example process, which may be performed by thecentral controller, for implementing the features described above. Thisprocess may be performed, for example, at a modem termination systemacting as the central controller. In step 701, the process may beginwith the central controller defining the various tones that will beused. This may include, for example, determining the frequency width,positioning and timing of the tones, such as the 10.5 kHz,100-microsecond long periods in the example above.

In step 702, the controller may initialize communications with thevarious devices (e.g., modems). This may be accomplished, in one type ofnetwork for example, via DOCSIS initialization of cable modems, throughwhich the CMTS learns of the presence of the various modems, such ascable modems, that are present on the network.

In step 703, the controller may initialize an adaptive modulation mapfor the various modems on the network. The initial adaptive modulationmap may include predetermined default values for the modems. Forexample, the modulation value may be initially set at the highestsupported modulation rate (e.g, 256-QAM). The controller may alsoinitialize an allocation map, which can identify the various tones andtheir usage, such as the times during which the tones are going to be inuse. For example, the allocation map might indicate that tone ‘B’ hasbeen reserved to modem ‘01342f33’ for time periods 11 and 12.

In step 704, the controller may transmit a pilot signal/ranging signalschedule to the various modems in the system. This schedule mayidentify, for each or a particular modem, the times and tones on whichthat modem is supposed to transmit a pilot signal and/or ranging signal.

In determining which tones to use as pilot or ranging signal tones, thecontroller may first avoid identifying any tones that have already beenallocated to a modem for upstream transmission (e.g., the controller maylimit its schedule to unused tones), and may also determine which tonesneed testing again (e.g., if a modem has never transmitted a pilot on aparticular tone, or if its last pilot on the tone was not successful,the controller may need that modem to try that tone again before tryinganother tone).

In step 705, the controller may begin to monitor the various tones thatwere supposed to carry pilot and/or ranging signals from modems, andcheck to see if there is a pilot/ranging tone that is expected and notyet checked for successful receipt. If a tone remains to be checked,then for the next tone, the controller may determine, in step 706,whether that tone was successfully received.

If it was, then the controller may update the adaptive modulation map instep 707 to indicate the success. This update may also includeupdating/generating information indicating the next tone and modulationlevel that the modem is to use for sending the next pilot/rangingsignal. The controller may store such information, and testing history,in the adaptive modulation map, and may use that information whenscheduling future pilot tones for the given modem. Or, as noted above,the modem can be programmed with a predetermined algorithm toautomatically determine the next modulation level to use based on thissame information, in which case any needed information may be suppliedfrom the controller as a response to the modem's pilot or rangingsignal.

In the case of a ranging tone, if the ranging tone is successfullyreceived, then the controller can provide information identifying thecharacteristics of the ranging signal that was received (e.g., time ofreceipt, phase, amplitude, etc.), and let the modem determine how bestto compensate for it. Alternatively, the controller may analyze thereceived signal and compare it against an expected signal (e.g., thepure carrier for the tone, or a predetermined data value that was to besent in the ranging tone), to identify the amount of distortion. Thisdistortion may then be used to computer the amount/type of linear and/ornonlinear predistortion needed to cancel out the distortion, andinformation storing this predistortion may be stored in the adaptivemodulation map as a distortion profile, and subsequently used toinstruct the modem to predistort its signal (e.g., the predistortiondata may be transmitted with the schedule in step 704). Examples of thispredistortion are discussed in greater detail below.

If the signal was successfully received, the process may then determine708 whether a request for allocation was included in the signal. If itwas, then the process may proceed to step 709, and one or more availabletones may be allocated to the requesting modem.

The allocation may be for a fixed duration (e.g., an allocation for tensymbol periods, 200 μs, 800 μs, etc.), and may be based on criteriaincluded in the modem's request. For example, the request may identifythe amount of upstream bandwidth desired by the modem, the duration ofthe bandwidth, and any requests regarding specific tones (e.g., if themodem wishes to request a particular tone) or modulation (e.g., if themodem wishes to use a particular modulation type). The controller maydetermine the tone(s) to allocate by consulting another allocation map,which may be a table listing the various tones and identifying the onesthat are in use (and other details, such as the duration of the use, themodem using it, the modulation on it, etc.), and identifying anavailable tone from the adaptive modulation map that can support thedesired modulation. For example, if the request indicates that the modemwould like to transmit at a rate of 100 bits/second, then the controllermay consult the allocation map to identify tones that are not already inuse, and the adaptive modulation map to identify available tones that,for the requesting modem, can support the required modulation level toprovide the requested data transmission rate. The central controller maythen prepare and transmit a downstream response 710 to the requestingmodem to acknowledge receipt of the signal, and to provide allocationdetails, such as allocated tone(s) and duration.

The response may be transmitted on a separate downstream channel fromthe upstream tones (e.g., DOCSIS downstream channel in one type ofnetwork), and may indicate whether the signal was successfully received,and/or the signal characteristics of the received signal (similar to theresponse to ranging signals, with the time of receipt, amplitude, phase,etc.). Additionally, the response may indicate the type of modulation totry next on this tone, although that information may alternatively beprovided as part of the schedule above, or the selection of the nextmodulation may be automatically performed according to a predeterminedalgorithm.

If, in step 706, the expected pilot and/or ranging tone was notsuccessfully received, then the controller may update the adaptation mapin step 711 with failure information. Such a failure update may indicatethe tone and modulation that was attempted, and may also include adetermination of what tone and modulation to expect in the nextpilot/ranging signal from the same modem. After processing the failure,the controller may also send a response to the modem, although thisresponse might simply indicate that no signal was received, or identifythe signal characteristics (timing, phase, amplitude, etc.) of whateversignal was actually received on the tone. In some embodiments, thefailure to receive a signal does not result in a response oracknowledgement message—the lack of such a response may be treated bythe modem as an indication that the signal was not received.

After transmitting a response (if any), the controller may then returnto step 705 to consider the next expected pilot tone, which may be froma different modem. If no more pilot/ranging tones are expected, then theprocess may proceed to step 712, and process the various tones that hadpreviously been allocated and which now carry upstream data. When theupstream data has been received, the controller may then update theallocation map in step 713 to remove allocations that are no longerneeded (e.g., the modems that have finished their transmissions, orwhose allocations have expired), and the process may return to step 704to begin another round of pilot/ranging signal scheduling andtransmission.

The process shown in FIG. 7 is just an example to illustrate concepts,and variations are certainly possible. For example, the illustratedprocess includes transmitting the schedule for each round ofpilot/ranging signals (e.g., once for each time period illustrated inFIG. 5). This is not required. As an alternative, the schedule could betransmitted less frequently (e.g., and include information and/oralgorithms scheduling signals for future time periods). Also, thesequence of steps may be changed. For example, the receipt of actualcontent on allocated tones (712) may occur simultaneously with thereceipt of the pilot/ranging tones. Other variations, such as theremoval, combination, rearranging, and/or addition of steps, may also beimplemented as desired.

FIG. 8 illustrates a process of using tones, from the perspective of theend device (e.g., the modem). In step 801, the device may firstestablish communications with the central controller, similar to step702. In step 802, the device may receive the schedule transmitted by thecontroller in step 704, and may prepare itself to transmit the pilot orranging signal information on the tone identified in the schedule, andusing the modulation identified in the schedule. This may involve, forexample, generating the pilot signal packet for transmission at theappropriate time.

In step 803, the device may determine whether it needs to transmitupstream data. If it does, then in step 804, it may determine the amountof need for the request. This may include, for example, identifying thedata transmission amount, rate, duration, type, etc. that is needed. Instep 805, the request and any needed request identification informationmay be inserted into the pilot signal that is about to be transmitted.In some embodiments, the device may be scheduled to transmit a pilotand/or ranging signal on every time period (e.g., continuously), so therequest can simply be added to the next outgoing signal withoutrequiring a separate contention to make the request.

In step 806, the device may transmit the next pilot, ranging, and/ordata signal at the designated time and using the designated modulation(according to the schedule).

In step 807, which may occur after sufficient time has elapsed for thecentral controller to respond, the device may check to see if the pilot,data, and/or ranging signal has been acknowledged by the controller. Theacknowledgement may arrive on any desired downlink channel, such as anMPEG stream in a DOCSIS downstream channel. The acknowledgement maycontain information identifying the characteristics of the signal thatwas received, to allow the device to measure the distortions that mayhave occurred in the signal path between the device and the centralcontroller. These distortions, and the device's ability to offset them,are discussed further below.

The acknowledgement may also include information responding to thedevice's request for allocation (if one was included in the originalpilot, data, or ranging signal). The response may include, for example,a simple indication that the request has been received and is beingprocessed, or it may include a full response to the request such as agrant identifying the tone(s) and time(s) allocated to the requestingdevice, or a denial of the request with information identifyingreason(s) for the denial.

If an acknowledgement has been received, then it may update its internalrecords in step 808 to reflect a successful transmission of the signal.This updating may include a determination of ramifications of thesuccessful transmission. For example, the end device may be tasked withdetermining the modulation to be used on the next pilot signal, and itmay make this determination in response to the acknowledgement. In someembodiments, both the central controller and the end device may have apredefined algorithm of changing modulation types, so that both thecontroller and the device know what modulation will be used next basedon the success/failure of the previous pilot, data, or ranging signal.For example, both may have a hierarchy of modulations (e.g., 256-Qam,64-Qam, 16 Qam, QPSK), and both may be configured to automaticallychoose, for a subsequent pilot or ranging signal transmission, the nextsequential modulation in the hierarchy in response to a success orfailure at the previous modulation. This chosen modulation may be thenext one used when the device is required to send another pilot orranging signal on the same tone.

A successful transmission may also result in a determination of theparticulars for the next scheduled pilot, data, or ranging signal (e.g.,which tone to use, when to send, etc.). For example, the schedule mayindicate that if one modulation is successfully used on the tone, thenthe next data signal will use the next higher modulation (unless thathigher modulation has recently failed). Alternatively, the schedule maysimply compare the successful modulation with a needs database (e.g.,comparing the successful modulation level with anticipated and/orhistorical need of the user to determine whether the successfulmodulation would be sufficient for this particular user, in which casethe next pilot signal would be best used to test a new tone instead oftesting a higher modulation level for the previous tone).

The main sources of interference tend to be from radio communications,such as short wave radio and impulse noise from electrical motors andlighting fixtures. These interference sources occur intermittently inboth time and frequency. When a data tone is transmitted, there is aprobability that interference at the time of transmission and at thefrequency of the tone is such that the receiver cannot demodulate thesignal correctly. If the signal is correctly demodulated then a positiveacknowledgement of reception is sent from the central controller to theend device. If the signal is not correctly demodulated by the receiver,then a negative acknowledgement is sent from the central controller tothe end device.

If the negative acknowledgment is received, then the transmitter canrepeat the data with a random delay, hoping that the interference sourcehas gone away. This can be repeated several times until a positiveacknowledgement is received. Each repeated transmission can add moreforward error correction to increase the chances of successfulreception. After several unsuccessful attempts, the data can be sent onanother tone that has been measured by a pilot tone to have betterchances of reception. In some embodiments, data will not be sent on atone that has many failed transmission attempts even at high forwarderror correction and low modulation rate, until a pilot tone has beenused to verify that the source of interference has gone away.

After a successful transmission, the process may return to step 802 toawait receipt of additional schedule information identifying thetone/modulation/time to be used for the next pilot, data, or rangingtone. In some systems, the previously-provided schedule may alreadyprovide this information, in which case step 802 may either be a checkfor an update/modification to that schedule, or be skipped altogether ifno new schedule is needed.

In no acknowledgement was received in step 807, then in step 809, thedevice may register a failure. The failure may also result in the devicechoosing a different level of modulation for the next time a pilot,data, or ranging signal is sent on the same tone. This dynamic changingof modulation level may use an automatic repeat request (ARQ) approach,and may be automatically carried out according to a predeterminedschedule (e.g., automatically drop one level when there is a failure;automatically drop one level when there are three failures in a row,etc.). The device may also use a higher amount of forward errorcorrection (FEC) in the next transmission of the pilot signal. In someembodiments, the device may continue to re-transmit the pilot signalpacket, at lower and lower levels of modulation and with greater andgreater levels of FEC, until it is acknowledged. In some embodiments, amodulation rate may be chosen to accept a certain amount of error (e.g.,a 10% error rate), so the schedule there may involve having the deviceretry a tone and modulation combination even after it is unsuccessfulthe first time.

The central controller may also note this failure, since it wasexpecting a pilot or ranging signal according to the schedule, but didnot receive one. The process may then return to step 802 to awaitadditional schedule information, as discussed above.

The discussion above has generally discussed each modem sending a singlepilot or ranging signal on a given tone. However, multiple modems maytransmit on a single tone, if desired, using any desired channel sharingtechnique. For example, a code division multiple access (CDMA) schememay be used to allow multiple modems to transmit on a single tone. Sucha CDMA scheme may, for example, allow multiple different cable modems totransmit signals on a particular tone. The schedule transmitted to themodems in step 704 may also identify how the modems are to share tones,such as by providing CDMA codes to the various modems. Frequencyhopping, time-division multiple access (TDMA) and other techniques mayalso be used.

FIG. 9 illustrates another example frequency map, similar to that shownin FIG. 5, but illustrating how the different tone modulations may beadapted for different time periods (not necessarily sequential) in thevarious tones of the upstream portion of the spectrum (in this example,and for illustration only, the 5-42 MHz range of a coaxial cable or anHFC network). The first example time period 901 represents an idealsituation, in which minimal interference exists and the cable modem isable to use all tones at the maximum modulation. That maximum modulationmay be, for example, an orthogonal frequency division multiple access(OFDMA) modulation scheme. The second example time period 902 mayillustrate a time in which greater interference exists. A lower form ofmodulation (e.g., QPSK) may be used at the upper and lower ends of thespectrum, and tones in between may have variations in performance. Inmany cases, lower frequency signals are more susceptible to noise andinterference, so the example time period 902 shows the modulation at thelower-frequency tones being lower than the modulation athigher-frequency tones.

As noted above, the time period used for the tones may be a value thatcorresponds with upstream burst periods used in DOCSIS, in one type ofexample network. For example, a 100-microsecond symbol period may be anideal duration to allow for a DOCSIS upstream burst to occur. To avoidinterference, the tones that are found in the DOCSIS upstream burstfrequency range may be selectively deactivated for the duration of theDOCSIS upstream burst. As illustrated in FIG. 9, the third time period903 shows a range of tones that corresponds with DOCSIS 3.0 upstreamburst traffic, so the central controller may simply deactivate (orrefuse to allocate) those tones for the duration of the DOCSIS burst.Tones lying in other frequency ranges, such as the tones in the twoOFDMA regions of period 903, may still be allocated for use. Similarly,in the fourth time period 904, tones that lie in the upstream DOCSIS 2.0burst, for example, may be deactivated as well, to allow those bursts tooccur. Older versions of DOCSIS, such as 1.1, may be treated in the samemanner. This coordination may be made easier by having the centralizedcontroller act as the DOCSIS CMTS.

Deactivation of tones may be made for other reasons as well. Forexample, burst noise situations may result in the system deactivatingranges of tones. Range 905 in FIG. 9 is an example of such tones beingdeactivated for a time period due to burst noise.

As noted above, ranging signals may be used to help offset distortionsoccurring in the signal path between the device and the centralcontroller. Due to the individually-controllable multitude of tones,ranging and predistortion may be handled for both the fundamental toneand its harmonics. FIG. 10 illustrates an example process for thispredistortion. In the FIG. 10 example, a ranging signal may betransmitted on a fundamental tone, at 0 dBmV, in step 1001 (transmittedas per step 806 above, for example).

The ranging signal may include a time value for the transmission, and avalue identifying the transmitting device and/or the tone on which theranging signal was initially transmitted. The central controller mayreceive the ranging signal in step 1002, and measure the amplitude andphase distortion of the fundamental tone and its received harmonics. Thecontroller may then instruct the device in step 1003 to increase theranging signal by 1 dB, for example, and send it again. The device cantransmit it again at the new level in step 1004, and the controller canonce again receive the signal and measure the amplitude and phasedistortion of the tone and its harmonics in step 1005. The controllermay check, in step 1006, whether the maximum strength (e.g., 60 dB) hasbeen tested. If it has, the process can end, but if it hasn't, theprocess can return to step 1003 to advance by another dB, and repeat.The controller can then store information identifying, for each modemand for each of the amplitudes, amplitude and phase distortionsexperienced by a signal from the modem to the controller. Thisinformation, which can be stored in a database as a nonlinear distortionprofile, may then be supplied to individual modems for use inpredistorting their future transmissions.

The multitude of tones advantageously allows the device to range notonly on the fundamental tone, but to also range on harmonics of thetone. Using a 5 MHz example, the steps may be repeated for the harmonics(e.g., tones on the 10 MHz, 15 MHz and 20 MHz harmonics of thefundamental 5 MHz tone). As a result, the device will have receiveddelay and amplitude/phase distortion information for the fundamentaltone and its harmonics.

When the device knows of the delay and amplitude/phase offsets (ordistortions) created by transmitting on a given fundamental tone, thedevice can use that information the next time it transmits data on thattone. When it does so, it can adjust the signal, or predistort it, tooffset (e.g., by adding a 180-degree opposite signal to theamplitude/phase distortion) the distortions introduced along the signalpath to the central controller. This predistortion may be added to thefundamental tone being transmitted, and other predistortions may also beadded to the harmonics of that tone, based on the distortions measuredon the harmonics. Predistorting the fundamental tone signal and theharmonics can help minimize overall distortions and allow for highermodulation and channel capacity. For example, if a different device wereto use a harmonic tone for transmitting data, the interference caused bythe first device's fundamental tone may be offset, allowing for acleaner signal on the harmonic for the different device.

FIG. 11 illustrates an example diagram of a modem having apre-distortion circuit, which can be used with embodiments describedherein. The components can be implemented using any desired RF ordigital components (which may involve use of an analog-to-digitalconverter if signals are processed in the digital domain). On the leftside, a signal (S) that the modem is preparing to transmit may be splitwith a three-way splitter 1101. One of the signals (the top one in FIG.11) may be supplied to a circuit 1102 that subjects the signal to thesame non-linear characteristics measured by the modem termination serverfor this modem during non-linear harmonic ranging. The output of thisdistortion circuit 1102 may be the original signal plus predistortion(S+D).

A second one of the signals (the middle one in FIG. 11) may be suppliedto a phase shifter, which can shift the signal (S) by 180° phase(illustrated as a negative one (−1)), resulting in an inverse of thesignal (−S).

The third one of the signals (the bottom one in FIG. 11) may be just acopy of the original signal (S).

The top two signals may be summed at a summing circuit 1104, resultingin a signal containing just the distortion (D) measured by the modemtermination server. This distortion (D) may be supplied to another 180°phase shifter 1105, resulting in an inverse of the distortion (−D). Thisnegative distortion (−D) can be combined with the original signal (S) ata summing circuit 1106, to result in an output signal containing theoriginal signal (S) and an inverse of the distortion (−D) that thesignal is expected to encounter on its way to the modem terminationserver.

When this signal hits the return path laser on its way to the modemtermination server, distortion components will be produced that are 180degrees out of phase with the distortion components artificiallyproduced in the cable modem, S−D+D=S. The distortion components canceland only the signal is present at the output of the optical receiver.

This can be adjusted in an adaptive closed loop system so that the modemtermination server measures errors and adjusts pre-distortion parametersto minimize the mean squared error. By doing this the carriers at 10,20, 30, and 40 MHz are all usable. Since a carrier can have as much as25 Mbps data throughput, this method could potentially add as much as 25Mbps of upstream capacity.

Simpler predistortion methods could also be used that have less benefit,but still provide substantial capacity increase. For example, by varyingthe modem transmit power and monitoring the harmonic distortion during aranging process, the optimum signal-to-noise ratio (SNR) input level tothe return path amplifiers and fiber node can been measured. Thus themodem termination server could always direct the modem to transmit atthe highest possible SNR. Since the return path characteristics changeover time and temperature and there is measurement uncertainty in themanual set up of the return path input levels, the automaticoptimization of the return path SNR can provide a significantimprovement in SNR and thus upstream capacity.

These are just two examples of how non-linear harmonic ranging andpre-distortion can be implemented. The nonlinear response of the returnpath can be determined by the composite signal of the superposition ofall carrier waveforms from all modems. The non-linear pre-distortioncould account for the composite waveform which would require precisioncontrol and timing from multiple modems and upstream carriers. Thepre-distortion circuit could account for just one dominant carrier whilerestricting other carriers to low levels that do not impact thenon-linear characteristics. The pre-distortion circuit could alsorequire that only one modem transmit at high power for specified timeslots, so that only the non-linear response from a single modem need beaccounted for in the pre-distortion circuit. The simplest method is tojust sum the average powers of all the modems and set the amplitude ofthe modems so that the average power into the return path laser is atthe highest level and yet still in the linear operating range.

The descriptions above are all illustrative examples. These features maybe combined, divided, rearranged, removed, augmented, and otherwisealtered as desired for any given implementation, and this patent shouldnot be limited to the specific examples described. To the contrary, thispatent should only be limited by the claims that follow.

1. A system comprising: a first device and a second device, wherein thefirst device comprises: one or more first processors; and first memorystoring instructions that, when executed by the one or more firstprocessors, cause the first device to: receive, from the second device,via a plurality of tones, and during a corresponding plurality of timeslots, a plurality of pilot signals; determine, for the second deviceand based on the received plurality of pilot signals, a plurality ofeffective modulation rates respectively associated with the plurality oftones; determine, based on the plurality of effective modulation rates,a tone, of the plurality of tones, for a subsequent transmission fromthe second device; and send, to the second device, informationindicating the determined tone, and wherein the second device comprises:one or more second processors; and second memory storing instructionsthat, when executed by the one or more second processors, cause thesecond device to: send, to the first device and based on a pilot signalschedule, the plurality of pilot signals; and receive, from the firstdevice, the information indicating the determined tone.
 2. The system ofclaim 1, wherein the instructions stored by the first memory, whenexecuted by the one or more first processors, cause the first device todetermine the tone by: determining that the determined tone has aneffective modulation rate, from amongst the plurality of effectivemodulation rates, that corresponds to a modulation rate requested by thesecond device.
 3. The system of claim 1, wherein the instructions storedby the first memory, when executed by the one or more first processors,cause the first device to determine the plurality of effectivemodulation rates by: determining, for each tone of the plurality oftones, a corresponding effective modulation rate of a pilot signal ofthe received plurality of pilot signals.
 4. The system of claim 1,wherein the instructions stored by the first memory, when executed bythe one or more first processors, further cause the first device to:after determining that a pilot signal of the received plurality of pilotsignals comprises an embedded tone allocation request: determine amodulation rate requested by the second device; and allocate, to thesecond device, one or more tones, of the plurality of tones, that havecorresponding effective modulation rates that support the modulationrate requested by the second device.
 5. The system of claim 1, whereinthe pilot signal schedule indicates representative tones of theplurality of tones on which the second device is to transmit each pilotsignal of the plurality of pilot signals, and wherein the representativetones skip one or more intervening tones in-between the representativetones.
 6. The system of claim 1, wherein the instructions stored by thefirst memory, when executed by the one or more first processors, causethe first device to determine the plurality of effective modulationrates by: inferring, based on effective modulation rates of tones thatare adjacent to at least one tone of the plurality of tones, aneffective modulation rate for the at least one tone.
 7. The system ofclaim 1, wherein the instructions stored by the first memory, whenexecuted by the one or more first processors, further cause the firstdevice to: determine one or more distortions in a pilot signal of thereceived plurality of pilot signals and on a tone of the plurality oftones; determine, based on the one or more distortions, a predistortionfor the subsequent transmission; and instruct the second device topredistort, in accordance with the predistortion, the subsequenttransmission.
 8. The system of claim 1, wherein the instructions storedby the first memory, when executed by the one or more first processors,further cause the first device to: determine one or more distortions ina pilot signal of the received plurality of pilot signals and on a toneof the plurality of tones; determine, based on the one or moredistortions, a predistortion for the subsequent transmission; andinstruct the second device to predistort transmissions on one or moreharmonic tones corresponding to the tone of the plurality of tones. 9.The system of claim 1, wherein the instructions stored by the firstmemory, when executed by the one or more first processors, further causethe first device to: after a failed attempt by the second device to senda pilot signal, receive, from the second device, a retransmission of thepilot signal, wherein the retransmission uses at least one of: a lowerlevel of modulation than the failed attempt, or a greater level offorward error correction than the failed attempt.
 10. The system ofclaim 1, wherein the instructions stored by the first memory, whenexecuted by the one or more first processors, further cause the firstdevice to: receive, from the second device, a sequential transmission ofpilot signals at each tone of the plurality of tones.
 11. The system ofclaim 1, wherein the pilot signal schedule indicates other time slots atwhich the second device is to send a plurality of ranging signals on theplurality of tones.
 12. The system of claim 1, wherein the instructionsstored by the first memory, when executed by the one or more firstprocessors, further cause the first device to: prevent transmission ofthe plurality of pilot signals via one or more tones, of the pluralityof tones, that are allocated to a third device.
 13. The system of claim1, wherein the pilot signal schedule is specific to the second device.14. The system of claim 1, wherein the pilot signal schedule indicatesthe tones and the time slots during which the second device sends theplurality of pilot signals.
 15. One or more non-transitorycomputer-readable media storing instructions that, when executed, cause:receiving, from a modem, via a plurality of tones, during acorresponding plurality of time slots, and based on a pilot signalschedule, a plurality of pilot signals; determining, for the modem andbased on the received plurality of pilot signals, a plurality ofeffective modulation rates respectively associated with the plurality oftones; determining, based on the plurality of effective modulationrates, a tone, of the plurality of tones, for a subsequent transmissionfrom the modem; and sending, to the modem, information indicating thedetermined tone.
 16. The one or more non-transitory computer-readablemedia of claim 15, wherein the instructions, when executed, cause thedetermining the tone by causing: determining that the determined tonehas an effective modulation rate, from amongst the plurality ofeffective modulation rates, that corresponds to a modulation raterequested by the modem.
 17. The one or more non-transitorycomputer-readable media of claim 15, wherein the instructions, whenexecuted, cause the determining the plurality of effective modulationrates by causing: determining, for each tone of the plurality of tones,a corresponding effective modulation rate of a pilot signal of thereceived plurality of pilot signals.
 18. The one or more non-transitorycomputer-readable media of claim 15, wherein the instructions, whenexecuted, further cause: after determining that a pilot signal of thereceived plurality of pilot signals comprises an embedded toneallocation request: determining a modulation rate requested by themodem; and allocating, to the modem, one or more tones, of the pluralityof tones, that have corresponding effective modulation rates thatsupport the modulation rate requested by the modem.
 19. The one or morenon-transitory computer-readable media of claim 15, wherein the pilotsignal schedule indicates representative tones of the plurality of toneson which the modem is to transmit each pilot signal of the plurality ofpilot signals, and wherein the representative tones skip one or moreintervening tones in-between the representative tones.
 20. The one ormore non-transitory computer-readable media of claim 15, wherein theinstructions, when executed, cause the determining the plurality ofeffective modulation rates by causing: inferring, based on effectivemodulation rates of tones that are adjacent to at least one tone of theplurality of tones, an effective modulation rate for the at least onetone.
 21. The one or more non-transitory computer-readable media ofclaim 15, wherein the instructions, when executed, further cause:determining one or more distortions in a pilot signal of the receivedplurality of pilot signals and on a tone of the plurality of tones;determining, based on the one or more distortions, a predistortion forthe subsequent transmission; and instructing the modem to predistort, inaccordance with the predistortion, the subsequent transmission.
 22. Theone or more non-transitory computer-readable media of claim 15, whereinthe instructions, when executed, further cause: determining one or moredistortions in a pilot signal of the received plurality of pilot signalsand on a tone of the plurality of tones; determining, based on the oneor more distortions, a predistortion for the subsequent transmission;and instructing the modem to predistort transmissions on one or moreharmonic tones corresponding to the tone of the plurality of tones. 23.The one or more non-transitory computer-readable media of claim 15,wherein the instructions, when executed, further cause: after a failedattempt by the modem to send a pilot signal, receiving, from the modem,a retransmission of the pilot signal, wherein the retransmission uses atleast one of: a lower level of modulation than the failed attempt, or agreater level of forward error correction than the failed attempt. 24.The one or more non-transitory computer-readable media of claim 15,wherein the instructions, when executed, further cause: receiving, fromthe modem, a sequential transmission of pilot signals at each tone ofthe plurality of tones.
 25. The one or more non-transitorycomputer-readable media of claim 15, wherein the pilot signal scheduleindicates other time slots at which the modem is to send a plurality ofranging signals on the plurality of tones.
 26. The one or morenon-transitory computer-readable media of claim 15, wherein theinstructions, when executed, further cause: preventing transmission ofthe plurality of pilot signals via one or more tones, of the pluralityof tones, that are allocated to a second modem.
 27. The one or morenon-transitory computer-readable media of claim 15, wherein the pilotsignal schedule is specific to the modem.
 28. The one or morenon-transitory computer-readable media of claim 15, wherein the pilotsignal schedule indicates the tones and the time slots during which themodem sends the plurality of pilot signals.
 29. An apparatus comprising:one or more processors; and memory storing instructions that, whenexecuted by the one or more processors, cause the apparatus to: receive,from a modem, via a plurality of tones, during a corresponding pluralityof time slots, and based on a pilot signal schedule, a plurality ofpilot signals; determine, for the modem and based on the receivedplurality of pilot signals, a plurality of effective modulation ratesrespectively associated with the plurality of tones; determine, based onthe plurality of effective modulation rates, a tone, of the plurality oftones, for a subsequent transmission from the modem; and send, to themodem, information indicating the determined tone.
 30. The apparatus ofclaim 29, wherein the instructions, when executed by the one or moreprocessors, cause the apparatus to determine the tone by: determiningthat the determined tone has an effective modulation rate, from amongstthe plurality of effective modulation rates, that corresponds to amodulation rate requested by the modem.
 31. The apparatus of claim 29,wherein the instructions, when executed by the one or more processors,cause the apparatus to determine the plurality of effective modulationrates by: determining, for each tone of the plurality of tones, acorresponding effective modulation rate of a pilot signal of thereceived plurality of pilot signals.
 32. The apparatus of claim 29,wherein the instructions, when executed by the one or more processors,cause the apparatus to: after determining that a pilot signal of thereceived plurality of pilot signals comprises an embedded toneallocation request: determine a modulation rate requested by the modem;and allocate, to the modem, one or more tones, of the plurality oftones, that have corresponding effective modulation rates that supportthe modulation rate requested by the modem.
 33. The apparatus of claim29, wherein the pilot signal schedule indicates representative tones ofthe plurality of tones on which the modem is to transmit each pilotsignal of the plurality of pilot signals, and wherein the representativetones skip one or more intervening tones in-between the representativetones.
 34. The apparatus of claim 29, wherein the instructions, whenexecuted by the one or more processors, cause the apparatus to determinethe plurality of effective modulation rates by: inferring, based oneffective modulation rates of tones that are adjacent to at least onetone of the plurality of tones, an effective modulation rate for the atleast one tone.
 35. The apparatus of claim 29, wherein the instructions,when executed by the one or more processors, cause the apparatus to:determine one or more distortions in a pilot signal of the receivedplurality of pilot signals and on a tone of the plurality of tones;determine, based on the one or more distortions, a predistortion for thesubsequent transmission; and instruct the modem to predistort, inaccordance with the predistortion, the subsequent transmission.
 36. Theapparatus of claim 29, wherein the instructions, when executed by theone or more processors, cause the apparatus to: determine one or moredistortions in a pilot signal of the received plurality of pilot signalsand on a tone of the plurality of tones; determine, based on the one ormore distortions, a predistortion for the subsequent transmission; andinstruct the modem to predistort transmissions on one or more harmonictones corresponding to the tone of the plurality of tones.
 37. Theapparatus of claim 29, wherein the instructions, when executed by theone or more processors, cause the apparatus to: after a failed attemptby the modem to send a pilot signal, receive, from the modem, aretransmission of the pilot signal, wherein the retransmission uses atleast one of: a lower level of modulation than the failed attempt, or agreater level of forward error correction than the failed attempt. 38.The apparatus of claim 29, wherein the instructions, when executed bythe one or more processors, cause the apparatus to: receive, from themodem, a sequential transmission of pilot signals at each tone of theplurality of tones.
 39. The apparatus of claim 29, wherein the pilotsignal schedule indicates other time slots at which the modem is to senda plurality of ranging signals on the plurality of tones.
 40. Theapparatus of claim 29, wherein the instructions, when executed by theone or more processors, cause the apparatus to: prevent transmission ofthe plurality of pilot signals via one or more tones, of the pluralityof tones, that are allocated to a second modem.
 41. The apparatus ofclaim 29, wherein the pilot signal schedule is specific to the modem.42. The apparatus of claim 29, wherein the pilot signal scheduleindicates the tones and the time slots during which the modem sends theplurality of pilot signals.